Thiol-terminated sulfur-containing polymers have a long history of use in aerospace sealants because of their fuel-resistant nature upon cross-linking. Among the commercially available polymeric compounds having sufficient sulfur content to exhibit this desirable property are the polysulfide polymers described, e.g., in U.S. Pat. No. 2,466,963 and sold under the trade name Thioplast® polysulfide (Akcros Chemicals, Germany); U.S. Pat. No. 4,366,307 sold in complete sealant formulations by PRC-DeSoto International, Inc. of Glendale, Calif.; and PCT/US01/07736, PCT/US01/07737, and PCT/US01/07738. In addition to fuel resistance, polymers useful in this context must also have the desirable properties of low temperature flexibility, liquidity at room temperature, high temperature resistance, a reasonable cost of manufacture, and not be so malodorous as to prevent commercial acceptance of compositions that contain the subject polymers.
An additional desirable combination of properties for aerospace sealants which is much more difficult to obtain is the combination of long application time (i.e., the time during which the sealant remains usable) and short curing time (the time required to reach a predetermined strength). Singh et al., U.S. Pat. No. 4,366,307, disclose such materials. Singh et al. teach the acid-catalyzed condensation of hydroxyl-functional thioethers. The hydroxyl groups are in the beta-position with respect to a sulfur atom for increased condensation reactivity. The Singh et al. patent also teaches the use of hydroxyl-functional thioethers with pendent methyl groups to afford polymers having good flexibility and liquidity. However, the disclosed condensation reaction has a maximum yield of about 75% of the desired condensation product. Furthermore, the acid-catalyzed reaction of β-hydroxysulfide monomers yields significant quantities (typically not less than about 25%) of an aqueous solution of thermally stable and highly malodorous cyclic byproducts, such as 1-thia-4-oxa-cyclohexane. As a result, the commercial viability of the disclosed polymers is limited.
Another desirable feature in polymers suitable for use in aerospace sealants is high temperature resistance. Inclusion of covalently bonded sulfur atoms in organic polymers has been shown to enhance high temperature performance. However, in the polysulfide polyformal polymers disclosed in U.S. Pat. No. 2,466,963, the multiple —S—S— linkages result in compromised thermal resistance. In the polymers of Singh et al., U.S. Pat. No. 4,366,307, enhanced thermal stability is achieved through replacement of polysulfide linkages with polythioether (—S—) linkages. In practice, however, the disclosed materials also have compromised thermal resistance due to traces of the residual acid condensation catalyst.
Yet another desirable feature of materials useful as aircraft sealants is the ability of the polymeric system to cure or cross-link under ambient conditions. For the purposes of this application, the term “ambient conditions” refers to temperatures and humidity levels typically encountered in an aircraft manufacturing environment. Numerous potentially useful cross-linking reactions occur at ambient conditions.
To achieve the required blend of application and performance properties, current commercial products are usually multi-component sealants. Widely accepted aerospace sealants consist of a first component containing at least one ungelled thiol terminated sulfur containing polymer and a second component containing either an oxidizing compound, e.g., manganese dioxide or any of a variety of Cr(VI) compounds, or a thiol reactive material, e.g. a polyepoxide, polyene or polyisocyanate. Optionally, both the first and second components also contain one or more formulating ingredients chosen from the list of fillers, pigments, plasticizers, stabilizers, catalysts, activators, surface-active compounds, solvents, and adhesion promoters. The types and quantities of these later ingredients are chosen and adjusted such that specific properties are achieved. Any of a number of other reactive groups, e.g., hydroxyl-, amine-, acryloxy-, siloxy- and maleimide, may be introduced onto the sulfur containing polymer backbone. By proper choice of ambient curing chemistry, these alternative reactive groups may be equally substituted for the aforementioned thiol functionality.
Polysulfide sealants may also be supplied in two-part injection kits, such as those manufactured by Techcon Industries and sold under the trade name Techkit, or in single component cartridges as premixed and frozen sealant (PMF). PMF sealants are highly desired by end users as they eliminate the tedious and costly process of on-site mixing as well as providing the end user with the convenience of a one-component material that has the fast cure rate typical of a two-component sealant.
Currently, the construction of both small- and large-scale aircraft sub-assemblies is entirely a manual operation. The quality and integrity of the sealed joint or seam is totally reliant on the skill and ability of the sealing operator to: correctly and thoroughly measure and mix the different sealant components; correctly apply the requisite amount of sealant for each bond-line requiring sealing; and reproducibly, over time, repeat this process on every sub-assembly requiring sealing.
In addition, it is highly desirable for each and every sealing operator to seal each and every sub-assembly in exactly the same manner. In practice, airframe manufacturers have addressed this challenge through comprehensive training programs, detailed sealing procedures and numerous in-process inspections. Despite these efforts, defects are common and resealing requires removing the part from the assembly-line process, an action that substantially diminishes manufacturing efficiency.
Aircraft manufacturers, in an effort to increase the fuel economy, attempt to reduce the weight of every aircraft component. Sealant manufacturers have responded to this demand for lower density sealants by incorporating lightweight fillers. The lightweight fillers suitable for use in aerospace applications may be classified as being organic, inorganic, or a hybrid composition. The particle morphology falls within two categories—hollow microspheres and amorphous filler particles.
The manufacture of premixed and frozen lightweight sealants is a relatively new, and not well understood, process. To those skilled in the art, the longstanding process for preparing a premixed and frozen polysulfide sealant involves the pumping of the base and curing agent components, in the proper mix ratio, into a rotary mixing chamber. The mixed sealant is then extruded into cartridges that are flash frozen to a temperature below the liquid sealant's glass transition temperature. This flash freezing process step is accomplished by immediately placing the sealant cartridge into a dry ice bath or into a liquid nitrogen freezing chamber.
Unexpectedly, when premixed and frozen lightweight sealants are prepared, it is known that many of the physical properties of the thawed sealant are degraded. Thus, there exists a need for a process to produce premixed and frozen low density aerospace sealants having resistance to mercaptan-containing aviation fuels. An additional need exists for a sealant that reduces labor and variability while affording ambient cure, shelf stability and light weight.